1. Field of the Invention
This invention relates to an electrophotographic process which adopts a potential control technique by control of a discharge quantity from a corona discharger to the photosensitive member. More particularly, it is concerned with potential control on a photosensitive member used in an electrophotographic reproduction apparatus, for example, wherein, when a rotational speed of the photosensitive member is varied in accordance with a magnification changing ratio.
2. Description of Prior Art
In order to form a latent image having a predetermined electric on a photosensitive member, the conventional potential control has been done in such a manner that a latent image having bright and dark patterns is first formed on the photosensitive member, a potential of the thus formed latent image is measured by a potential sensor, and a corona discharge quantity to be applied to the photosensitive member is varied until a desired potential value is reached. Variations in this corona discharge quantity are so controlled that the latent image having both bright and dark patterns may be converged on a predetermined electric potential. For this purpose, the conventional potential control method should repeat over a plurality of numbers of times those steps of: experimental latent image formation, potential detection, and change in current applied to a discharger, and, as soon as conditions have been fixed, they are held in a holding circuit. Further, the potential control in the conventional electrophotographic method is also used for converging a varying electric potential on a predetermined target value.
In the following, explanations will be made as to a case, wherein a moving speed of the photosensitive member, i.e., the process speed, is varied for image reproduction in a changed magnification.
Heretofore, the potential control has been done with fixed constants (such as target potential, initial value, control factors, etc.) necessary for the potential control on the basis of, for example, an equal magnification (1:1 scale) as a standard, and, even when the process speed should be changed at the magnification changing mode, it has been done with such fixed constants. Even with such control method, the potential control can be done for either case of the equal magnification and changed magnification, since the potential contrast can be constantly controlled irrespective of the process speed. Practically, however, if an initial value is fixed, an initial charge quantity varies with change in the process speed with a consequence that a longer time than in the case of the equal magnification is taken until a predetermined potential is reached. In general, those conditions such as a bias voltage value at the side of the developing means, a number of revolutions of a developing sleeve, and others are fixed. As the result, a difference would come out in the developed images due to variations in the process speed, even if the latent image contrast is constant. For instance, at the time of magnification, or enlargement, the process speed becomes lower than a standard value and the charge quantity increases substantially. Therefore, when the initial charge quantity is constant, a longer time than at the standard process speed is required for an increase in this initial charge quantity, until the potential control is terminated and a constant contrast is obtained. Also, in case a constant contrast is reached in a state of the process speed being slowed down, if the developing conditions are constant, the image quality becomes such that its density is higher and the background is stained with more fogging than in the case of the standard process speed, because the latent image is subjected to more development for a portion of the slowed-down speed. In the case of scale-reduction, on the contrary, since the process speed becomes increased, the initial charge quantity becomes lower than in the case of the standard process speed. This leads to prolongation of the time until completion of the potential control same as mentioned above. Furthermore, when the potential of the photosensitive member has reached a predetermined constant contrast, the image quality will be such that its density is generally low and an intermediate color tone tends to run out, if the development conditions are constant, because of increased process speed. For such reasons, when the process speed is variable, it is inconvenient to maintain the control constants fixed.